Electric contact



July l, 1941" F. R. Hl-:NsE-L :TAL 2,247,755

ELECTRIC CONTACT Filed Fqb'. 3, 1940 FP6/rz R. HEMS'EA ATTORNEY Patented July 1, 1941 2,247,755 f ELECTRIC CONTACT Franz E. Hensel andv Kenneth L. Emmett, In-

dianapolis, Ind., assignors to P. R. Mallory i Co., line., Indianapolis, Ind., a corporation of Delaware Application February 3, 1940, Serial No. 317,135

(Cl. 20G- 166) 8 Claims.

This invention relates to electric make-andbreak contacts of refractory metals.

An object of the invention is to improve electric contacts.

Other objects of the invention will be apparent from the following description taken in connection with the appended claims.

The present invention comprises the combination of elements, methods of manufacture, and the product thereof brought out vand exemplied in the disclosure hereinafter set forth, the scope of the invention being indicated in the appended claims.

lWhile a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made inthe method of procedure and the combination of elements without departing from vthe spirit of the invention.

In the drawing Figure 1 is a side view of an electric contact disc embodying features of the invention; and

Figure 2 is Ia cross-section of the contact.

The present invention contemplates an electric contact having a. body of refractory metal such as tungsten, molybdenum and thelike and a contacting surface of a hard metal of improved contact characteristics such as rhodium. The

contact surface metal is preferably diffused into the refractory body at the junction,

Tungsten contacts have been extensively used heretofore and have given relatively good service in contact applications where substantial contact pressures are available. However, up to the present time tungsten contacts have not .been entirely satisfactory for operation at low contact pressures due to objectionable metal transfer and the tendency of the material to become oxidized. This results in high contact resistance at low pressures and often causes erratic operation. For this reason the use of tungsten contacts in sensitive meters and. recording equipment, telephone circuits and similar systems has generally been unsatisfactory due to the-objectionable high resistance and to the production of microphonic noises in the circuits. The provision of a satisfactory contact material for such applications is extremely difficult due to the low contact pressures encountered and Athe require ment that the contacts operate satisfactorily after long periods of use or non-use. Such 'contacts are also often required to stand up satisfactorily in corrosive atmospheres such as in atmospheres where a certain amount of sulphur compounds are present.

Standard tungsten contacts .capable of operat-v ing at current values of from 1 to 10 milliamperes at from 1/2 to 6 volts and atpressures of from 60 to 150 grams are usually found to havel contact resistance values between the contact pair of from .5 to 3 milliohms under these conditions. Under low contact pressure conditions, however, such as under pressures' from a fraction vof a gram to 5 grams, such contacts will have contact resistance in the magnitude of from 50 to 300 milliohms. Such resistances are often objectionably high in the circuits of sensitive or selective equipment and where low voltages are used.

The objections discussed pronounced with molybdenum contacts.

Applicants have now discovered that contacts of tungsten or molbydenum can be improved greatly and made available for low pressure and low .voltage contact operation by applying a relatively thin surface of rhodium to the contact and preferably by diffusing the rhodium into the contact material a certain amount. The diffusion greatly improves the bond between the two metals and prevents separation or spalling on of the surface material.

According to our preferred method of carrying out the invention a contact disc of tungsten or molybdenum cut from rod or punched from a flat strip of the material is given a thin coating of rhodium on its contact surface. Rhodium may be applied in any one of several ways, the most important of which are described below:

vThe cleaned tungsten -or molybdenum disc may be plated with a. layer of rhodium by a process briefly as follows: v

1. The refractory'metal disc is electrochemically cleaned in a solution of sodium hydroxide and tri sodium phosphate, followed by rinsing in 10% aqueous hydrochloric acid, followed by a rinse in hot Water.

2. The discs are then plated in a standard acid rhodium plating bath sufficient to give 2 lgrams of rhodium per liter, current densities of approximately 50 amperes persquare foot being used.

After plating the plated contact may be subjected to a diffusion process to obtain partial diffusion of the rhodium into the underlying tungsten or molybdenum. 4The simplest and preferred method of obtaining such diiusion is by the application of external heat. For example, the plated contact disc may be placed in a furnace in a reducing atmosphere. The rate above are even moreV and degree of diffusion will, of course, depend on the time, temperature, composition and thickness of the layer to be diffused. 'Ihe temperature can be varied from 750 to 1950" C. depending upon the condition of the surface to which the plate is applied and the extent of diffusion desired. The diffusion is preferably carried to a point where the rhodium diffuses into the backing metal a small distance, such as .0005 to .010". There should remain a thin layer of substantially pure rhodium at the outer surface, which layer may have a thickness in the order of .0001 to .0002" although greater thicknesses are also possible by the process. The composition gradually changes from substantially pure rhodium at y the surface to pure tungsten or molybdenum at a short depth below the surface, the composition in the intervening region varying with the depth below the surface. i

According to a modified method the refractory metal disc is first plated with an extremely thin layer of rhodium, after which the heat treatment is applied to produce partial or complete diffusion into the refractory metal. The plating process is then repeated to apply an additional layer of rhodium and further diffusion is produced by heating. This sequence may be repeated several times until the desired thickness is built up.

In some instances it is also possible to apply a surface layer consisting of an alloy formed predominantly of rhodium and containing relatively lesser amounts of elements such as palladium, platinum and iridium. Several ways of accomplishing this have been found feasible. It is possible for instance to provide a surface consisting of a rhodium-platinum alloy by electroplating the refractory metal contact base first with a thin layer of one of the elements and then a thin layer of the other and then applying a heat treatment to cause diusion of the two plated layers into each other and at the same time into the refractory base material. Here likewise several platngs may `be individually applied interspersed with heat treatments.

Methodsl other than plating can be used to apply the contact surface metal to the refractory base. For example, the surface layer can be applied by cathode sputtering, metal spraying or by condensing evaporated metal onto the surface. Other methods involve the deposition of the surface metal in finely divided form onto the refractory metal surface followed by heat treatment to produce a diffused surface layer. For example, the metal may be deposited from colloidal suspensions of the finely divided metal either by a settling process or by electrophoretic deposition. The refractory metal surface may likewise be coated with a dispersable adhesive and the powder metal dusted or sprayed onto the surface, after which the heat treatment is applied to disperse the adhesive and diffuse the coating layer into the surface. The metal may also be applied as a paste containing the metallic powders which is printed onto the contact surface. 'Ihe finely powdered coating metal can also be pressed into the surface in a suitable press. In some cases a metal compound such as metallic oxide or a volatile compound of rhodium such as the chlorides, borides or iodides may .be used. In the case of the oxide the compound is reduced by heating in a reducing atmosphere leaving the coating on the surface. In the case of the other materials 'a reaction with the contact material backing takes place forming a strong adherent bond of the desired thickness.

A diffused layer of rhodium on a tungsten surface has been found to have an extremely high hardness and high resistance to corrosion. The hardness of rhodium matches closely the hardness of tungsten and tungsten ls therefore an ideal base metal. In addition to contact discs the base metal may be in the form of rivets, strip, screws, wire or any other suitable form.

Negligible wear was observed after-prolonged periods of operation of the contacts, the operation apparently having a desirable polishing effect upon the contact surfaces.

The diffused rhodium surface layer appears practically unattackable by corrosive atmospheres at ordinary temperatures. The contacts are also relatively free from electrolytic disintegration which is often encountered in contacts of the prior art. Contacts according to the present invention are particularly useful for microphone switches, relay apparatus for automatic locking of elevator gates, automatic safety signals, voice circuit controls in isolated substations, radio and sound reproducing apparatus especially Where the presence of sulphur in vulcanized rubber is liable to produce tarnish on ordinary contacts, communication receivers, sensitive recording meters and automatic telephone contacts. The contacts are also useful for many applications Where tropical climates are encountered.

The contact base material used is preferably dense non-porous tungsten or molybdenum such as is obtained from swaged and heat treated tungsten or molybdenum rod.

Referring to the drawing, Figure 1 is a side view of a contact having a tungsten or molybdenum body I0, a rhodium surface H and intervening region l2 wherein the rhodium is diffused into the tungsten or molybdenum. Figure 2 is a cross-section of the same contact.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. An electric contact having a body formed of a refractory metal and a contact face thereon formed predominantly of rhodium.

2. An electric contact having a `body formed of a refractory metal and a contact face thereon formed predominantly of rhodium, said contact face being integrally bonded to said refractory metal body.

3. An electric contact having a body formed of' a. refractory metal and a contact face thereon formed predominantly of rhodium, said contact face being integrally bonded to said refractory metal body and said facing metal being diffused into said refractory body metal.

4. An electric contact having a body of refractory metal selected from the group consisting of tungsten and molybdenum and a. contact face thereon formed predominantly of rhodium.

5. An electric contact having a body of refractory metal selected from the group consisting of tungsten and molybdenum and a contact face thereon formed predominantly of rhodium, said contact face metal being integrally bonded to and diused into said refractory body.

t 6. An electric contact having a body of refractory metal selected from the group consisting of tungsten and molybdenum and a rhodium` contact face thereon.

7. An electric contact having a body of refractory metal selected from the group consisting of tungsten and molybdenum and a rhodium contact face thereon, said rhodium being integrally bonded to and diffused into said refractory metal body.

tungsten body.

FRANZ R. HENSEL.

KENNETH L. EMLIERT. 

